1. Field of the Invention
The present invention relates to a transistor device, display device, and a method of fabricating a display device, and more particularly, to a thin film transistor device, a liquid crystal display (LCD) device, and a method of fabricating an LCD device.
2. Discussion of the Related Art
In general, an LCD device comprises a thin film transistor (TFT) array substrate, an opposing substrate, and a liquid crystal layer between the TFT array and opposing substrates. The TFT array substrate includes TFTs used as switching devices in the LCD device. The TFTs are generally used in active matrix type liquid crystal display (AMLCD) devices for laptop computers. In the AMLCD device, each of the TFTs is formed at a crossing point of gate and data lines, wherein the gate and data lines cross each other to define a plurality of pixel regions. Accordingly, the TFTs function as switches for turning ON or OFF a current to the pixel region. More specifically, in the turning ON state, a capacitor of the pixel region is charged to a predetermined voltage level by the current. Meanwhile, in the turning OFF state, the pixel region is maintained as the charged state until the unit pixel region is addressed to a next state. In this state, the voltage level determines light transmittance through liquid crystal corresponding to the unit pixel region, thereby determining a gray level.
FIGS. 1A to 1G are plan and cross sectional views of a fabrication process of a TFT array substrate according to the related art. In FIG. 1A, a plurality of gate lines 12 and gate electrodes 12a are formed on a lower substrate by a photolithographic process that includes deposition of a metal layer having low resistance on a transparent glass substrate having great heat-resistance at a high temperature, and deposition of a photoresist thereupon. Then, a mask having a predetermined pattern is positioned on the photoresist, and light is selectively irradiated thereto, thereby forming the same pattern as that of the mask on the photoresist. Next, some portions of the photoresist irradiated with light are removed and patterned by using an etchant, and the metal layer having no photoresist thereon is etched, thereby obtaining a desired pattern of the photoresist. For example, the etching method is classified into a dry-etching method using plasma, and a wet-etching method using chemical solution.
In FIGS. 1B and 1C, an inorganic layer of silicon nitride SiNx or silicon oxide SiOx is deposited along an entire surface of the lower substrate 11 including the gate line 12 at a high temperature, thereby forming a gate insulating layer 13. Subsequently, an island-shaped semiconductor layer 14 is formed on the gate insulating layer 13 above the gate electrode 12a. At this time, the semiconductor layer 14 is formed in a method of depositing an amorphous silicon (a-Si:H) at a high temperature and patterning the amorphous silicon by photolithography.
In FIGS. 1D and 1E, a metal layer is deposited along an entire surface of the lower substrate 11 including the semiconductor layer 14, and patterned by a photolithographic process, thereby forming a data line layer. The data line layer includes a data line 15 perpendicular to the gate line 12 to define a pixel region, and source and drain electrodes 15a and 15b overlap with both sides of the semiconductor layer 14. At this time, the gate line 12, the gate electrode 12a, the data line 15, and the source and drain electrodes 15a and 15b are formed of a low-resistance metal material, such as aluminum Al, aluminum neodymium AlNd, molybdenum Mo or chrome Cr. The deposited gate electrode 12a, the gate insulating layer 13, the semiconductor layer 14, and the source and drain electrodes 15a and 15b form a thin film transistor.
In FIGS. 1F and 1G, an organic insulating layer of BCB (BenzoCycloButene) is deposited along an entire surface of the lower substrate 11 including the data line 15, thereby forming a passivation layer 16. Then, the passivation layer 16 is selectively removed, so that it is possible to form a contact hole exposing the drain electrode 15b. Next, a transparent conductive layer of ITO (Indium-Tin-Oxide) or IZO (Indium-Zinc-Oxide) is formed along an entire surface of the lower substrate 11 including the passivation layer 16, and then patterned by a photolithographic process, to form a pixel electrode 17 contacting the drain electrode 15b in the pixel region, thereby completing the TFT array substrate.
FIG. 2 is a cross sectional view of an LCD device according to the related art. In FIG. 2, the lower substrate 11 having the TFT is bonded to an upper substrate 21. The upper substrate 21 includes an R/G/B color filter layer 23 corresponding to the pixel regions to display various colors, a black matrix layer 22 that excludes light leakage in the portions defining the pixel regions, and a common electrode 24 opposite to the pixel electrode. Like the lower substrate 11, the upper substrate 21 is formed of the transparent glass substrate having great heat-resistance, since the process of forming the pattern is carried out at the high temperature.
Although not shown, ball spacers (not shown) of plastic or silica, each having a predetermined diameter, are uniformly formed between the lower and upper substrates 11 and 21, to maintain a cell gap between the two substrates 11 and 21. In addition, a sealant (not shown) is printed along a circumference of an active region to prevent liquid crystal material from leaking, and to bond the two substrates together. For example, the sealant is printed with a screen mask having a predetermined pattern, wherein the sealant is not printed in the portion corresponding to an inlet through which liquid crystal material is injected. Then, the inside of the two substrates is maintained in the vacuum state, and the liquid crystal material is injected between the two substrates 11 and 21 by using a capillary phenomenon and a pressure difference, thereby forming a liquid crystal layer 31. Thereafter, the inlet is sealed, so that it is possible to complete the LCD device.
In order to form the aforementioned LCD device, the process must be performed at a temperature between 250° C. to 400° C. For example, the gate insulating layer 13 and the semiconductor layer 14 are deposited using a plasma enhanced chemical vapor deposition (PECVD) method. In this case, the above-mentioned deposition process is carried out at a temperature 250° C. or more. Accordingly, a glass substrate having great heat-resistance is used for the LCD device. However, the glass substrate is heavy, requires the complicated fabrication process, and has low mobility.
To solve these problems, a flexible substrate has been actively studied to form a display device having lightness in weight, great shock absorption and flexibility. Recently, instead of the glass substrate for the LCD device, use of a plastic substrate is on an increasing trend. However, a coefficient of thermal expansion (CTE) of the plastic substrate is ten times as high as a CTE of the glass substrate, so that it requires a low temperature process, such as a process at below 150° C. or less. Thus, it is necessary to form the line layer, the semiconductor layer, and various insulating layers using organic material, since the organic material is suitable for the low temperature process.
According to the related art, 7 masks are required for forming the gate line layer, the semiconductor layer, the data line layer, the contact hole of the passivation layer, the pixel electrode, the black matrix layer, and the color filter layer. Thus, the fabrication process is complicated due to the increase of the number of masks. As a result, the fabrication efficiency deteriorates by the increase of fabrication cost and time.